1. Field
The present writing is directed to GaN-based sensor nodes for in situ detection of gases.
2. Related Art
The AlGaN/GaN hetero structures contain polarization sheet charges (i.e. a two dimensional electron gas) at the interfaces due to differences in the piezoelectric effect of AlGaN and GaN. These polarization charges respond very sensitively to the exposure to polar liquids and gas molecules. In the AlGaN/GaN high electron mobility transistors (HEMT), the ions created by the reaction of chemical molecules with the transition metal contacts induce significant effects on the electrostatic surface termination of the nitride and thus influence the barrier height of the Schottky contacts. In fact, several experimental results published in recent literature indicate that AlGaN/GaN HEMT has a strong potential for chemical sensor. See, for example, “Gas Sensitive GaN/AlGaN-heterostructures” by Schalwig J, Muller G, Eickhoff M, Ambacher O, Stutzmann M, Sensors and Actuators B-Chemical, 87 (3): 425-430 Dec. 20 2002; “Group III-nitride-based Gas Sensors for Combustion Monitoring” by Schalwig J, Muller G, Eickhoff M, Ambacher O, Stutzmann M, Materials Science and Engineering B-Solid State Materials for Advanced Technology, 93 (1-3): 207-214 May 30, 2002; “GaN-based heterostructures for sensor applications” by Stutzmann M, Steinhoff G, Eickhoff M, Ambacher O, Nebel CE, Schalwig J, Neuberger R, Muller G, Diamond and Related Materials, 11 (3-6): 886-891 March-June 2002; “High-Electron-Mobility AlGaN/GaN Transistors, (HEMTs) for Fluid Monitoring Applications” by Neuberger R, Muller G, Ambacher O, Stutzmann M, Phys. Status Solidi A 185, 85 (2001); and “GaN-based diodes and transistors for chemical. Gas biological and pressure sensing” by Pearton S. J., Kang B. S., Kim S., Ren F., Gila B. P., Abernathy C. R., Lin J., Chu S. N. G., J. Phys.: Condens. Matter 16 (2004) R961-R994.
However, a limitation of AlGaN/GaN hetero structure devices, similar to other chemical sensing FETs (field effect transistors), is poor selectivity in chemical detection. The low selectivity in chemical detection is a major hurdle to practical approach.
Further, a majority of the current stand-off or remote chemical detection/monitoring systems are based on optical/IR spectral analysis and deployed on mobile platforms. These systems offer excellent chemical identification capability but are large, expensive, and require long data acquisition times.
Another major drawback of optical/IR systems is that functionality can be dramatically reduced in bad weather/fog due to undesirable signal attenuation. In addition, for the safety and security of the end users, real-time and autonomous monitoring of the continuously changing chemical environment using remote sensing networks is also very important.